Apparatus for controlling a power control element of a drive unit of a motor vehicle

ABSTRACT

An arrangement for driving a power control element of a drive unit of a motor vehicle is proposed and the arrangement has a full-bridge output stage. A direct-current motor is arranged in the bridge diagonal and the current through the direct-current motor is limited to at least one preadjusted value in that the clocked drive signals are interrupted when the preadjusted value is exceeded.

FIELD OF THE INVENTION

The invention relates to an arrangement for controlling a power controlelement of a drive unit of a motor vehicle.

BACKGROUND OF THE INVENTION

Such an arrangement is known form U.S. Pat. No. 4,951,188. There, afull-bridge circuit is provided for controlling a power control element,especially a throttle flap in the context of an electronic acceleratorpedal system. The full-bridge circuit comprises four actuable circuitelements which can be driven by at least one pulse-shaped drive signal.The consumer, the electric motor, is arranged in the bridge diagonal andis connected to the power control element. Two diagonally opposite lyingswitch elements are closed for actuating the motor in a first directionof rotation in the sense of opening the power control element. The twoother diagonally opposite lying switch elements are actuated whenactuating in the second direction. The so-called free-running phase isin contrast to these current flow phases in which the drive signalexhibits an active level. In the free-running phase, the drive signalexhibits an inactive level and none of the switch elements is driven. Amean current is generated by the consumer because of the clocked drivingof the power control element and the clocked control comprises thecurrent flow phase and the free-running phase. The mean current holdsthe consumer against a return force in the position pregiven by theelectronic accelerator pedal system.

In order to protect the output stage against short circuits, a measuringresistor in the region of the ground connection of the full bridgedetects the total current flowing through the entire bridge and throughthe consumer. This current is compared to a pregiven maximum value and,when this maximum value is exceeded, the control is reduced to apulse-duty factor of 1% and is resumed when there is a drop below thepregiven maximum value. A current value is fixed as the maximum currentvalue and this current value is that current which would flow throughthe bridge when there is a short circuit in the consumer. The knownoutput stage is protected against short circuits because of thereduction of the drive when the threshold value is exceeded. By reducingthe drive to a pulse-duty factor of 1%, intense current fluctuationsoccur in the case of a short circuit when the threshold is exceeded orthere is a drop below the threshold which would cause an unsatisfactoryperformance of the control system.

The mechanics of the control element and the control apparatuselectronics must be matched to each other when driving a power controlelement, especially a throttle flap, with a direct-current motor. Inthis context, opposing requirements occur. Pregiven maximum displacementtimes for the power control element are to be maintained and an excessof torque of the drive is to be made available in the case of binding oricing of the control element. The direct-current motor should be held ina pregiven position with the lowest possible current; the smallest peakloads in the vehicle electrical system are to be generated and, finally,the driving output stage should be configured so as to be protectedagainst short circuits and be integratable in a simple manner; that is,the maximum current required by the output stage should be as low aspossible.

Not all of these requirements are satisfied in the known output stagewhich is secure against short circuits. A maximum current up to theshort-circuit current is permitted for the known output stage in orderto obtain a displacement time which is as short as possible. This alsoleads to a high excess torque. However, the direct-current motorutilized generates high currents or current peaks when changing thedrive control especially during displacement operation during itsacceleration and braking. In this way, the electrical system of themotor vehicle is greatly loaded so that large collapses in voltage anddrops can be the consequence. Furthermore, a very power intensive outputstage, which can only be integrated with difficulty, is required withwhich corresponding costs are associated.

The known output stage therefore cannot satisfactorily fulfill theopposing requirements made on an output stage for driving adirect-current motor.

If an output stage of lower power would be utilized for controlling theelectric consumer or motor, then the known short circuit protectionwould intervene and quasi switch off the drive for normal displacementas a consequence of current or voltage peaks when driving the consumeror displacing a direct-current motor.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an arrangement fordriving a power control element of a drive unit wherein the opposingrequirements are satisfied as best as possible and wherein especiallythe load on the vehicle electrical system is as low as possible withoutother characteristics of the drive system (especially the necessarydisplacement time) being significantly affected.

The arrangement of the invention defines a full-bridge output stage fordriving a direct-current motor wherein, on the one hand, the ability towithstand short circuits is provided by monitoring the currents throughthe individual transistors of the output stage with respect to exceedinga maximum short circuit current value and switching off the output stagein the event of a short circuit and, on the other hand, the currentthrough the full-bridge output stage is limited to a pregiven limitvalue and, in order to limit the current, the pulse-shaped drive signalis so clocked that exceeding the limit value is prevented. This limitvalue is then in amount less than the peak value of the current peaks,which occur during normal operation during the displacement operation,and is especially less than the switch-on peak and the reverse currentpeak. Accordingly, the result is then a limiting of the current throughthe consumer to the pregiven limit value in every operating state andnot only in the case of a fault. The limit current is held within narrowlimits.

U.S. Pat. No. 4,901,366 discloses that two opposite lying switchelements are actuated in the free-wheeling phase of a full-bridge outputstage for driving a power control element of a drive unit.

The above-mentioned opposing requirements are satisfied by the procedureaccording to the invention.

The opposing requirements are connected in an optimal manner with eachother by the procedure of the invention. This is so because the currentlimiting causes the load on the vehicle electrical system to be held aslow as possible during displacement operations without reducing thedisplacement time significantly. The control element mechanics (gearing,return springs, motor) can be optimized with a view toward displacementtime and holding current.

Furthermore, it is advantageous that the output stage itself is notdesigned with respect to the maximum permissible current of the motorbut with respect to the pregiven current limit value. Accordingly, theoutput stage does not require so much power and is therefore lessexpensive.

A clear maximum current is pregiven in the area of the output stage andin the electric motor. Accordingly, smaller wire cross sections in thearea of the output stage and of the direct-current motor can be selectedwhich likewise reduces costs.

High current peaks during the displacement operation are avoided by theprocedure of the invention so that drops in voltage in the controlapparatus supply are essentially avoided. The load on the vehicleelectrical system is thereby significantly reduced.

Furthermore, it is possible to save cost with respect to thedirect-current motor and to design the same smaller.

The procedure of the invention has special significance because of thesimple integratibility of the output stage. The integration of thecomponent is only possible because of the use of components of lowerpower and the limitation of the current flowing through the component asa consequence of reduced power loss. Also, extensive cooling measuresfor the component are not needed.

The subject matter of the invention is especially significant forguaranteeing operational reliability. Limiting the current through theconsumer to the limit value counters fault currents, especiallyshort-circuit currents, which arise because of short circuits flowingthrough the electric consumer (short circuit toward plus). Thesemeasures afford no protection for a short circuit, for example, of aconsumer terminal to ground. For this reason, the current through theindividual transistors is monitored which, in the case of a fault withthe occurrence of a short circuit, transfers the output stage into asafe condition. The reaction comprises, for example, limiting thecurrent through the transistor or switching off the output stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in greater detail with respect to theembodiments shown in the drawing. FIG. 1 shows a block circuit diagramof a control system for driving a power control element for which theprocedure of the invention is realized. FIG. 2 made up of 2aand 2b showsthe operation of the procedure of the invention compared to conventionalcontrol systems with the aid of signal traces. FIG. 3 shows theprinciple of the short circuit protection of the short-circuit proofoutput stage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an overview circuit diagram of a control system for drivinga power control element wherein the procedure of the invention isrealized. 10 identifies a computer element which has the input lines 12to 14. These lines connect the computer element 10 to measuring devices16 to 18. Furthermore, the computer element 10 has output lines 20 and22 which lead to a drive circuit 24. The output line 20 leads to a logicunit 25 to which lines 22 and 96 are connected. The four output lines32, 34, 36 and 38 of the logic unit 25 lead to respective switchelements of a full-bridge output stage 40.

In a preferred embodiment, the logic unit 25 includes a first logiccircuit 26 to which line 20 is connected and the output line 28 of thelogic circuit 26 leads to a logic arrangement 30. The logic unit 25 alsoincludes a second logic circuit 42 to which line 22 is connected and theoutput line 44 of logic circuit 42 is likewise connected to the logicarrangement 30.

The full-bridge circuit comprises a first switch element 46 to which theline 32 is assigned. The first switch element 46 is connected via line48 to the positive pole 50 of the operating voltage. On the other hand,the switch element 46 is connected via a line 52 to a second switchelement 54 to which the line 38 is assigned. The switch element 54 isconnected via the line 56, the measurement resistor 58 as well as theline 60 to the negative pole 62 of the operating voltage. Furthermore,the full-bridge circuit 40 has a third switch element 64 which isconnected via the line 66 to the positive pole 50 of the operatingvoltage and via the line 60 to the fourth switch element 70. The switchelement 70 is connected via the line 52, the resistor 58 and the line 60to the negative pole 62 of the operating voltage. The line 34 isassigned to the switch element 64 and the line 36 is assigned to theswitch element 70. A line 74 leads from line 68 to the terminal 76 ofthe circuit arrangement 24; whereas, a line 78 leads from the line 52 tothe terminal 80 of the circuit arrangement 24. The electric motor 82 ispreferably a direct-current motor and is connected between the terminal76 and 80. The direct-current motor is connected via a mechanicalconnection 84 to the power control element 86 which is especially athrottle flap. A line 88 leads from line 60 to detect the current, whichflows through the full-bridge circuit, to the comparator 90. The otherinput of the comparator is connected to the line 92 and this line 92connects the comparator 90 to a reference voltage source 94. The outputline 96 leads to the logic unit 25 where it is connected to both logiccircuits 26 and 42.

The computer element 10 forms output signals for adjusting the powercontrol element or the motor 62 in dependence upon the operatingvariables detected by the measuring devices 16 to 18. These operatingvariables are preferably accelerator pedal position, throttle flapposition, engine rpm, engine temperature, et cetera. The computerelement 10 forms the output signals preferably in the context of aposition control. These output signals are outputted to the drivecircuit 24 via the line 20 for the first direction of rotation and viathe line 22 for the second direction of rotation. The output signals arein the form of pulse-shaped signals and, in the preferred embodiment,the signals are pulsewidth modulated signals. The pulse width determinesthe duration for which the motor is supplied with current and therebythe adjustment of the power control element. This pulse width modulatedsignal is conducted over the corresponding line to the first logiccircuit 26 and the second logic circuit 42 and is there compared to thelogic output signal of the comparator 90. The logic output signal of thecomparator 90 is supplied via the line 96 to the logic circuits. Thelogic circuits 26 or 42 are configured in that the pulsewidth modulatedsignal is inverted and the inverted pulsewidth modulated signal iscoupled to the comparator output signal level via a logic NOR-function.The logic circuit must then satisfy the function that a high signallevel is present on the output line 28 or 44 when the pulsewidthmodulated pulse signal exhibits a high signal level but with thecomparator output signal level being low. In all other cases, the signalon the line 28 or 44 exhibits a low level. Other connections are alsoadvantageous in addition to the above-mentioned realization.

The signal level on the line 96 is formed by comparator 90 by comparingthe voltage dropped across measurement resistor 58 to the referencevoltage generated by element 94. This reference voltage is so fixed thatthe voltage value corresponds to a pregiven limit value of the currentflowing through the consumer and the bridge. If the voltage across themeasurement resistor 58 exceeds the reference voltage value, then thisindicates a current exceeding the pregiven limit value. The outputsignal level of the comparator 90 then changes over to a high level. Ifthe comparator output exhibits a high signal level, then the logiccircuits 26 or 42 block a possibly present high signal level of thepulsewidth modulated signal so that the drive is interrupted so longuntil the comparator again changes its level, that is, the voltageacross resistor 58 drops below the reference value.

The logic arrangement 30 receives the signals supplied via the lines 28or 44 and, in correspondence to the incoming signals, selects thecorresponding lines for actuating the switch elements of the full-bridgeoutput stage. For a positive signal level on the line 28, a current flowis triggered in the forward direction in that the elements 64 and 54 areclosed via the lines 34 and 38; whereas, for a negative signal level onthe line 28, the free running phase is assumed and either all switchelements are opened or, in a preferred embodiment, the switch elements64 and 46 are closed via the lines 34 and 32 so that the current doesnot flow back into the vehicle electrical system; instead, the currentfree runs in the bridge. In a corresponding manner, the logic circuit 30treats the signals on the line 44. A positive signal level on the line44 indicates an actuation of the motor in the rearward direction sothat, for a current flow phase, the switch elements 70 and 46 are closedvia the lines 32 and 36. Here too, the free-running phase is assumedwhen a low signal level is on the line 44 and all switch elements areopened; or, in the preferred embodiment, the switch elements 64 and 46are closed so that the current does not flow back into the vehicleelectrical system.

If the voltage drop across the measurement resistor 58 exceeds theadjusted value, then the output stage is transferred into thefree-running phase and the drive is interrupted so that, finally, thecurrent flowing through the motor 82 is limited to the preadjustedvalue. As soon as the dropped voltage value drops below the adjustedvalue then the signal level changes at the output of the comparator sothat the output stage is again enabled and the positive signal level ofthe pulse width modulated signal again becomes effective in supplyingthe flow of current and the motor is correspondingly actuated.

In addition to the illustrated preferred embodiment, negative logic canbe utilized in lieu of the positive logic in other embodiments, that is,the flow of current phase can be triggered by a low level of the drivesignal on the lines 20 and 22 and/or on the lines 32, 34, 36 and 38;whereas, the free-running phase is assigned to high signal level.Furthermore, the comparator 90 can have a hysteresis.

The principle of the short-circuit protection utilized is shown in FIG.3. Each of the switch elements 46, 54, 64 and 70 includes means fordetecting the current flowing through the circuit elements, respectively(for example, by utilizing so-called current-sensing FETs). If thecurrent flowing through an individual switch element exceeds a shortcircuit current threshold value, then at least this switch element isswitched off. This measure with reference to the short-circuitprotection of the output stage is independent of limiting the currentflowing through the output stage. This is shown by the example of theswitch element 46 having the transistor 100 with current detection 102.The detected current is conducted via a line 104 to a comparator 106.There, the current is compared to the preadjusted current value Imax(the short-circuit current) and an output signal is generated when thedetected current exceeds the limit value. The output line 108 of thecomparator and the drive line 32 of the switch element 46 are broughttogether in a NAND-gate 110. The drive of the transistor is interruptedin the case of a short circuit when the short-circuit current value isexceeded. The interaction between this current limiting by theindividual switch elements and the current limiting by the entire bridgeoccurs because the current through the consumer is limited to the limitvalue when fault currents occur and especially short-circuit currentswhich occur because of short circuits through the electrical consumer(short circuit to plus). This measure affords no protection, forexample, when a consumer is clamped to ground in a short circuit (forexample, a short circuit through transistor 64 when control driven). Forthis reason, the current through the individual transistors is monitoredand the output stage is transferred to a safe state in the event of afault from the occurrence of the short circuit. The reaction, forexample, is defined by a limiting of the current through the transistoror in a switchoff of the output stage.

The switch-on and reversing current peaks are capped during adisplacement operation with the procedure of the invention of additionalcurrent limiting a short-circuit protected output stage. This isachieved without a significant loss on displacement time.

The foregoing is shown with reference to the signal traces in FIG. 2 forthe example of controlling a direct-current motor 82 from a firstposition to a second position. In FIG. 2a, the time is along thehorizontal and the position α of the power control element (the throttleflap) is along the vertical. In FIG. 2b, the current flowing through themotor is along the vertical. It is here assumed that the motor is in aposition α₁ and is held in this position by a current I₁. At time pointT₀, a jump-like change of the drive signal occurs in the sense of aclosure of the throttle flap. This causes an abrupt increase in currentwhich, in an embodiment without the invention would have reached thevalue I₂ (shown by broken line). With the invention, this current islimited as shown by the solid line, for example, to 5 A so that theswitch-on peak, shown dotted, is avoided. Thereafter, the motor is setin motion so that the current drops until the motor has reached itsdisplacement speed. At time point T₁, the motor is braked because itapproaches its new position α₀. This leads to a counter voltage which,but for the invention, would result in the current peak I₃, for example,-15 A. With the procedure of the invention, even this reverse currentpeak is limited, for example to -5 A or another value. Thereafter, thecurrent slowly assumes the holding current which lies in the range ofthe previous current value. FIG. 2a clearly shows that the displacementof the motor, which is obtained by the procedure of the invention,departs only insignificantly from the displacement curve shown dottedwithout the procedure of the invention. A significant reduction of thedisplacement time cannot be discerned.

In addition to the embodiment shown for the control of a throttle flap,the procedure of the invention can also be utilized in combination withthe control of an injection pump lever of a diesel engine.

Furthermore, the comparator 90 can be mounted with measuring resistance58 and the reference voltage source 94 outside of the output stagecomponent.

Furthermore, the procedure of limiting current according to theinvention can also be applied to other circuit realizations of afull-bridge output stage with a drive circuit. Short circuit measuresare not shown for reasons of clarity; however, they can of course berealized in the context of the output stage.

In addition to the input of a limiting value for positive and negativecurrents, two limit values can be provided in an advantageous manner,for example, for displacing the throttle flap into the closed direction,a larger current can be permitted.

The limit values can, in an advantageous manner, also be pregiven independence upon operating variables such as engine temperature, coiltemperature, ambient temperature, position of the throttle flap, etcetera. For example, and in an advantageous embodiment, the currentlimitation can be lifted by the computer via a line (not shown) inspecific operating situations, for example, when the throttle flapbecomes jammed or is frozen tight.

Limiting the current in accordance with the invention is then soconfigured that, in time average, a current, which corresponds to thepredetermined limit value, flows through the consumer and the outputstage.

We claim:
 1. An apparatus for controlling a power control element of adrive unit of a motor vehicle, the apparatus comprising:an electricmotor for controlling said power control element; a source for supplyinga current; a full-bridge output circuit connected to said source and tosaid electric motor for supplying said current to said electric motor;said full-bridge output circuit having four arms; four switching unitsconnected into said four arms, respectively; pulse generating means forgenerating at least one pulse-shaped drive signal for driving saidswitching units for allowing said current to flow through saidfull-bridge output circuit and to said electric motor; limit means forproviding a limit value for said current; means for comparing saidcurrent to said limit value and for operating on said pulse generatingmeans to inhibit said pulse-shaped drive signal when said currentexceeds said limit value and for enabling said pulse generating meanswhen said current drops below said limit value; and, each one of saidfour switching units including short-circuit monitoring means formonitoring current through said one switching unit and for immediatelyswitching off said full-bridge output circuit when the current throughsaid one switching unit exceeds a predetermined limit value.
 2. Theapparatus of claim 1, wherein said pulse-shaped drive signal is apulsewidth modulated signal.
 3. The apparatus of claim 2, wherein saidmotor has two rotational directions and said full-bridge output circuitis driven by said pulse-generating means for the two rotationaldirections of the motor and a free-running phase.
 4. The apparatus ofclaim 3, further comprising means for interrupting a current flow phasewhen at least one preadjusted limit value is exceeded during the currentflow phase and initiating a free-running phase wherein all of saidswitching units are opened or two opposite lying switching units areclosed.
 5. The apparatus of claim 1, wherein said electric motor is adirect-current motor and said current through said direct-current motorcan be plotted as a function of time to provide a current trace having apeak value; and, said limit value for said current is less than saidpeak value in normal operation when said direct-current motor isdisplaced.
 6. The apparatus of claim 1, wherein said current can flowthrough said motor in a predetermined current direction; and, said limitvalue of said limit means is pregiven in dependence upon at least one ofsaid current direction and operating variables.
 7. The apparatus ofclaim 1, further comprising: a computer element incorporating means fornot considering the limit value of said limit means in predeterminedoperating situations.
 8. The apparatus of claim 1, wherein saidcomparator means has a hysteresis with reference to the current valuesbetween interrupting and resuming the drive of said electric motor. 9.The apparatus of claim 1, wherein said monitoring means in each of saidswitching units is adapted to compare the current through said oneswitching unit to a threshold value and at least switch off said oneswitching unit corresponding thereto when the threshold value isexceeded.
 10. The apparatus of claim 1, wherein the full-bridge outputcircuit is integrated into an output stage component and the limiting ofthe current is adjusted by external circuits of said full-bridge outputcircuit.
 11. The apparatus of claim 1, further comprising a computerelement which includes means for increasing said limit value of saidlimit means.